Selective Nerve Cuff Stimulation Strategies for Prolonging Muscle Output

Abstract

Neural stimulation systems are often limited by rapid muscle fatigue. Selective nerve cuff electrodes can target independent yet synergistic motor unit pools (MUPs), which can be used in duty-cycle reducing stimulation paradigms to prolong joint moment output.

Objective: This study investigates waveform parameters within moment-prolonging paradigms and determines strategies for their optimal implementation.

Methods: Composite flat-interface nerve cuff electrodes (C-FINEs) were chronically implanted on feline proximal sciatic nerves. Cyclic stimulation tests determined effects of stimulation period and duty cycle in different MUP types. Ideal parameters were then used in duty-cycle reducing carousel stimulation. Time to 50% reduction in moment (T50), moment overshoot, and moment ripple were determined for constant, open-loop carousel, and moment feedback-controlled closed-loop carousel stimulation.

Results: A stimulation period of 1 s best maintained joint moment for all MUPs. Low (25%) duty cycles consistently improved joint moment maintenance, though allowable duty cycle varied among MUPs by gross muscle and fiber type. Both open- and closed-loop carousel stimulation significantly increased T50 over constant stimulation. Closed-loop carousel significantly decreased moment overshoot over the other conditions, and significantly decreased moment ripple compared with open-loop stimulation.

Conclusion: Selectivity-enabled carousel stimulation prolongs joint moment over conventional constant stimulation. Appropriate waveform parameters can be quickly determined for individual MUPs and stimulation can be controlled for additional performance improvements with this paradigm.

Significance: Providing prolonged, stable joint moment and muscle output to recipients of motor neuroprostheses will improve clinical outcomes, increase independence, and positively impact quality of life.

Figure 1:

An example of carousel stimulation through a selective nerve cuff with three independent synergistic motor unit pools. Stimulating through one nerve cuff contact at a time causes some motor units to create a joint moment while inactive motor units rest and recover.

Figure 2:

The C-FINE electrode. A 16-channel C-FINE electrode in open (left) and closed (right) conformations.

Figure 3:

A schematic of the experimental setup. The hindlimb is fixed in a stereotactic frame with the hip, knee, and ankle joints at 90°. The paw is secured to a metal shoe attached to a 6 degree of freedom load cell, which measures ankle moments produced by C-FINE stimulation.

Figure 4:

An example of data from each phase of the cyclic stimulation protocol. The potentiation phase (1), the potentiated twitch response (2), and the cyclic parameter testing phase (3).

Figure 5:

The closed-loop carousel stimulation scheme. A carousel control signal c(t) determines which contact is stimulated through at each timepoint. Independently tuned PID controllers adjust the instantaneous PW sent through the active contact based on real-time joint moment feedback from the load cell. PW is adjusted to minimize the error e(t) between target and actual moment output.

Figure 6:

Sample moment space trajectories from the present study and their muscle classifications (MG, LG/Sol, TP, CP) as determined by comparison with Leventhal et al. [19]. Each colored line shows the trajectory achieved through stimulating a different cuff contact in this study. Note that there are five selective trajectories and at least two contacts that produce each trajectory. These contacts are further analyzed for overlap to see if they are recruiting the same MUPs or functionally redundant but unique MUPs that can be used for fatigue reduction.

Figure 7:

Plantarflexion moment summation of low (%OL = 1.4) and high (%OL = 48) overlapping contact pairs. The difference in actual moment summation from ideal summation (solid orange) shows extent of fiber overlap. Error bars signify standard deviation.

Figure 8:

Examples of the three twitch profiles used to classify MUPs into fiber type categories. Typical fast, mixed, and slow twitch profiles. A twitch contraction time cutoff of 45 ms (vertical dotted line) is used to classify MUPs as fast or slow [31]. Presence of absence of a second peak is used to classify mixed and pure fast fibers respectively.

Figure 9:

(Top) Example data showing normalized moment output from cyclic stimulation trials on the same MUP with different stimulation periods. Delta, the within-contraction change in moment, is indicated for the final contraction of each period by the symbol Δ_T. (Bottom) Mean moments (solid blue) and mean deltas (dashed grey) for stimulation periods of T = 1, 5, and 10 seconds after 1 minute of active stimulation at a 50% duty cycle. Each data point represents the mean value for a single trial. Error bars indicate standard deviation. A period of T = 1 second maintains significantly (p < 0.015) higher moments with significantly less within-contraction moment decline.

Figure 10:

The normalized mean moment outputs over 1 minute of active stimulation for 33 MUPs tested at each duty cycle. Error bars signify standard deviation. Lower duty cycles resulted in significantly (p<0.01) greater moment maintenance.

Figure 11:

Mean moment output (a) by predicted muscle and (b) by fiber type at different duty cycles. Error bars indicate standard deviation. Duty cycles sufficient to maintain significantly higher moment outputs after the same active stimulation time vary among muscles and fiber types.

Figure 12:

Performance of ASPs compared to constant and open-loop stimulation for three different groups of contacts: (a) c = 2, T = 1, target moment = 50 N-cm (b) c = 3, T = 1, target = 40 N-cm (c) c = 3, T = 1, target = 40 N-cm. (Left) Raw ASP moment traces are shown in faded blue and red traces for open and closed-loop trials respectively. Mean moment traces averaged over each full cycle through contacts are overlaid as darker solid lines. (Right) Feedback controller PW output to each contact during closed-loop carousel trials.